The present invention relates to electrostatic copiers, and more particularly to fusing electrostatic toner material to the surface of a copy sheet bearing toner. There are a number of well known devices that fuse or fix toner material to copy sheets by applying a combination of heat and pressure to a toner bearing copy sheet through a pair of relatively rigid rollers, one or both of which are internally heated in order to soften and thereby fuse the toner to the copy sheet. Other means and devices externally heat one or both rollers which are coated with special, thermally conductive, elastomeric material.
Still another device passes the copy sheet through the nip of a feed roller and thermally transparent, rigid drum. Inside the drum are a radiant energy lamp and a reflector which focuses the thermal energy on a line transverse to the direction of sheet travel. One such device is described in U.S. Pat. No. 3,452,181.
Heat assisted fusing devices pose a potential fire hazard in the event of either a sheet jam or mechanical component failure within the fuser. Failure to eject a fused copy sheet from the fixing rollers is normally detected by suitable jam sensing means. Such detector systems usually sense the interruption of the normal copy paper sheet flow through the fusing system. Any abnormal signal resulting from improper sheet flow to, through, or from the fusing station provides the jam detection system with a signal to shut down the fuser heating source. However, even when the jam detector system functions properly, the large thermal masses comprising the desired fusing devices may retain sufficient thermal energy to ignite any inadvertently jammed copy sheet or the like. Similarly, a focused energy source from a lamp could also readily ignite a jammed copy sheet before the lamp is extinguished and the resulting heat harmlessly dissipated.
Other problems associated with heated roller fusing devices are found with the tendency of the fused copy sheet to stick to the heated roller fusing surface immediately after the fusing process has been completed at the pressure roller pair. The internally heated rollers associated with present fusers are normally several inches or more in outside diameter. Unfortunately, this relatively large size associated with the heated rollers provides an opportunity for the fused copy sheet to naturally adhere to the roller surface due to slightly tacky toner material still warm from heat transfer. If even one sheet of copy paper sticks to the heated roller surface by virtue of slightly tacky toner, a potential fire hazard looms. Also, the fire hazard increases with the possible accumulation of fuel added by a stream of copy sheets being fed through the copier at relatively high lineal velocities. Accordingly, there have been many attempts to provide adequate jam sensing devices for copy sheets moving towards, through, and away from fusing devices. There has been additional effort directed at providing fire extinguishing means in many of the previous fusing devices known in the field today. However, a relatively high likelihood of fire hazard remains with the use of these fusing devices in the event of mechanical malfunction or paper jam occurences.